5 sourced reports, with what to check before you buy. Each one links to the page it was reported on.
Costly or dangerous
Hotend thermal runaway error #13204 on MK4 (thermistor/heater wiring)
Prusa's own knowledge base documents a MK4-specific thermal runaway error (#13204) that stops heating when the nozzle temperature drops 12°C and does not recover within 60s; it is traced to a damaged hotend heater or thermistor and the xBuddy fuse.
What to check. A buyer should expect a thermal-runaway shutdown to be a real, documented MK4 failure mode. It is safety-critical (heating stops) and the fix is checking/replacing the thermistor, heater, or the hotend fuse on the xBuddy board, which means opening the electronics.
Firmware corruption error #13608 on MK4 (internal flash)
Prusa documents a MK4-specific 'FW in internal flash corrupted' error (#13608) that appears when a firmware flash fails, leaving the printer unable to boot normally; recovery requires reflashing firmware/bootloader from a FAT32 USB drive.
What to check. A firmware update that fails can leave the MK4 showing this error and needing a manual USB reflash. It is recoverable but requires a PC, a FAT32 USB stick, and following Prusa's reflash procedure, so a buyer should be comfortable with that.
Hotend/nozzle clog on MK4 (Nextruder gears skip and grind filament)
Prusa's own MK4 hotend-clog article explains that a clogged nozzle/hotend makes the Nextruder drive gears skip, producing a clicking sound and grinding the filament; it also notes filament debris builds up in the feed gear grooves over time, causing under-extrusion.
What to check. Extruder clogging is a documented MK4 failure mode that shows as clicking and ground filament, and the fix involves cleaning the Nextruder gear grooves and clearing the nozzle — routine but hands-on maintenance the buyer should expect.
Power Panic error #13321 on MK4 (PP cable from PSU to xBuddy)
Prusa documents a MK4-specific Power Panic error (#13321) that appears at initialization when the Power Panic cable running from the power supply to the xBuddy board is damaged, and instructs checking that cable connection.
What to check. The Power Panic cable is a known electrical failure point that triggers an error at boot; checking it means inspecting the wiring at the PSU and xBuddy board, a simple but real repair a buyer may face.
Recurring first-layer too low / auto-leveling calibration issue on MK4
A French owner reports a recurring MK4 problem where the automatic first-layer leveling consistently prints the first layer too low, persisting across recalibrations, firmware updates, nozzle changes, and belt adjustments; raising Z manually by 0.010mm visibly fixes it.
What to check. This is an owner-reported first-layer calibration defect that resists Prusa's own support and normal recalibration, so a buyer may have to live with manual Z-offset tweaks per print or chase the issue with Prusa support.
These reports come from the pages linked under each item: owner forums, repair guides and workshops. They are not a diagnosis of any one unit, and a careful check of the one you are buying still applies.
Text from the Kriko 3D printers catalog, version 0.1.1, under the CC-BY-SA-4.0 license.
